1. Field of the Invention
The present invention relates to an apparatus for optically detecting photosensitive material (such as photographic film for prepress use) which is transported through an automatic processor, automatic developing machine or the like.
2. Description of the Related Art
Conventionally, an automatic processor has optical sensors disposed at a suitable location (such as at a photosensitive material inlet) for detecting photosensitive material. Signals from such sensors are used for the following controls:
(1) To change the photosensitive material transport mechanism from a relatively slow standby state to a selected processing speed,
(2) To increase drying temperature from a low standby temperature to a selected temperature,
(3) To start supplying wash water, and
(4) To measure the length of the photosensitive material by detecting the leading and trailing ends thereof, to derive the area of the photosensitive material from its length, and to replenish the machine with processing solutions on the basis of the area of the photosensitive material.
When optically detecting photosensitive material, it is necessary to prevent fogging of the photosensitive material due to the light emitted by the optical sensors. Examples of countermeasures against such fogging are disclosed in Japanese patent applications laid open under Nos. 1988-12986 and 1988-157087.
Such detecting apparatus prevent fogging of the photosensitive material by employing optical sensors that emit pulsed light so that cumulative exposure of the photosensitive material is maintained below a critical exposure level, which is determined by the sensitivity of the material. However, the pulse width of light emitted from such optical sensors is selected without regard to the photosensitive material transport speed. This gives rise to the following problem:
Generally, in an automatic processor, the photosensitive material transport speed may be changed for different types of photosensitive material. Therefore, the optical sensors emit pulsed light with a pulse width which is long enough to avoid fogging even when the photosensitive material is transported at a minimum speed.
However, this long pulse width cannot be used to precisely detect photosensitive material transported at a high speed. This is because the photosensitive material will advance a considerable distance from a point of time the photosensitive material reaches a position opposed to the optical sensors to a point of time the photosensitive material is detected by the pulsed light. Normally, the automatic processor has a maximum transport speed which is 3 to 6 times the minimum transport speed, and hence a large detecting error will occur at the maximum transport speed. This could lead to a serious situation, including improper developing, particularly when processing solutions are replenished on the basis of the measured area of the photosensitive material.